Greater Ice Storm Trap (4e Trap)

From D&D Wiki

A magical object taking up one square, such as an never melting ice pillar with a giant hailstone on the top, which causes a storm of bitterly cold hailstones to pummel a wide swath of ground and cover the area in ice. Once the pillar is destroyed the hailstone will fall and break, harmlessly shattering into pieces of ice.
Players can make an Knowlage Arcana Check, DC 31 to identify it when they see it. Being unable to move, it has a low reflex and is very easy to hit, but being made of solid ice means it has high AC and Fortitude and it is hard to land a damaging hit against those defenses.

This could be used in an encounter by positioning it so that players are within range while fighting monsters, making the encounter more difficult as they must try to avoid the Ice Storm attacks. This would work particually well in combination with monsters that are immune to cold damage, as the burst will not affect them.